The invention relates to a flash butt welding installation for metal bands passing successively along a longitudinal axis.
In metallurgical installations, in particular dedicated to the production of metal bands, bands must often be welded one after the other by electric welding, for example at the inlet of hot rolling band pickling facilities, at the inlet of continuous-continuous type cold roll mills and at the inlet of so-called xe2x80x98coupledxe2x80x99 facilities that perform pickling and cold rolling of sheets on a single line.
Generally, for continuous working, the downstream end must be welded, in the running direction, of a band whose processing is nearly finished, to the upstream end of the following band. The welding is then made along a transversal line, generally perpendicular to the running axis.
Various means are used to this end. For example, the ends of both bands can be applied one over the other and they can be welded by discharging electric current between two wheels running transversally.
But, in some cases, both bands must be butt-welded to avoid any excessive thickness. Both ends must therefore be shorn off, respectively downstream and upstream of both bands, then brought closer to one another so that the facing edges contact each other, in order to be flash welded.
Such a facility comprises hence various tools working in succession.
First of all, the ends, respectively downstream and upstream of the first and of the second bands are shorn by a cutting tool enabling to provide two edges, respectively upstream and downstream, at the ends of both bands that are fixed, during the shearing, by two retaining members.
Then, both shorn edges are brought close to one another and flash welded.
To this end, a welding facility comprising two pairs of clamping jaws is used, whereas the said jaws are mounted respectively on a fixed frame and on a mobile frame, movable longitudinally in order to bring both bands closer to one another, electric supply means creating a potential difference between the facing ends of both bands in order to weld them.
Moreover, the ends of the bands should preferably be applied one against the other under a certain pressure, to produce a forging effect of the welding.
However, the welding bead forms, on both faces of the band, burrs that must be removed so that the welded portion can travel through the various sections of the processing line. The machine comprises therefore usually a tool for burring and trimming the welded point, whereas the said tool consists most of the times of a grinding wheel or a trimmer fitted with knives that cut the burrs off, by moving transversally along the welded joint.
The machine must also be fitted with a number of appended devices, for example, one or several looker devices thereby providing a certain clearance of longitudinal displacement of each band without affecting the running means, centering means for perfect alignment of both bands, as well as a notched for removing the ends of the welded joint on the lateral edges of both bands, in particular when the said bands have not the same width.
To adjust the parallelism of the shorn edges and to determine their spacing in relation to the thickness of the bands to be welded, a spacing bar is often used, that is interposed between both edges that are applied on either side of this bar. These adjustments increase the duration of a welding operation and, moreover, call for the intervention of an operator.
Such installations are therefore rather complicated and require too much space, since the tools are used one after the other according to the order of operations to be performed.
Various arrangements have been proposed to simplify the operations and to perform them rapidly, with sufficient accuracy.
In particular, to reduce the number of manoeuvres, it has been suggested to perform all the operations without moving the bands, while using double shears built into the machine and performing simultaneously two perfectly parallel edges on both bands that remain clamped, throughout all the operations, between the jaws of both retaining members, respectively fixed and mobile.
For example, in the arrangement described in the document FR-A-2311626, double guillotine shears are used, interposed between the clamping jaws for cutting the ends of both bands simultaneously and then moved sideways to enable a welding tool to come forward, consisting of two sets of jaws forming power supply electrodes. These electrodes are installed, with the shears, on a frame that can move perpendicularly to the running axis of the band and that also carries a burring tool that, in a third position, flattens the welded joint.
Both these mobile electrodes are therefore power-supplied by contact with both retaining members, after longitudinal clamping of the assembly.
Generally speaking, the design of these electric circuits also raises numerous problems, considering the powers to be implemented, the space requirements of the tooling sets and the operating conditions.
For example, the sheets to be butt-welded being heated by a Joule effect, they must obviously be subjected to sufficient voltage to bring the required energy during the flash welding process and this voltage must be adjusted in relation to the sizes, thickness and width, of the sheets to be welded, whereas these sizes may, besides, vary according to the manufacturing programme.
However, it has been noted that the value of the supply voltage should also be limited in order to reduce the formation of oxides at the interface during the welding operations since these oxides tend to make the welded joint brittle.
Usually, the electrodes are supplied with alternate current using a transformer associated with means for setting the voltage applied.
A transformer capable of generating the necessary power is rather cumbersome and, in the arrangement of the document FR-A-2311626, it is located beneath the running plane of the bands, on a circulation track provided in the foundations. But consequently, the supply circuits and the isolation means of the electrodes can be damaged, during the shearing and welding operations, by falling chips, the projections of metal caused by the flash welding process and welding debris.
In order to remedy such shortcomings, it is possible to place the transformer above the running plane of the band according to an arrangement that has been divulged, for instance, in the document EP-A-0845309.
That document relates to a perfected welding machine that advantageously, with respect to the previous arrangement, does away with mobile electrodes, whereas the power required for welding is provided directly by the retaining members of both bands that are isolated with respect to the remainder of the machinery. Any contact resistance between the mobile electrodes and the clamping jaws is thereby avoided. Moreover, such a machine enables very accurate adjustment of the cantilever distances between the end of each clamping jaw and the corresponding shorn edge of the band.
Thanks to this arrangement, the circuits travelled by the current, notably because of their geometry, exhibit rather large impedance, which increases the power to be supplied for the welding process and, consequently, the voltage necessary to the flashing and the heating of the bands at a temperature enabling the welding and forging of the joint.
Still, we have seen that excessive voltage exhibits shortcomings and the installations used until now are therefore supplied with alternate current at low voltage, ranging for instance between 6 and 20 volts efficient. However, the voltage is adjusted by a terminal device that does not enable actual regulation of the welding realisation conditions. Moreover, this device must be motor-driven, that complicates the realisation of the transformer.
The invention relates to improvements made to the welding installations enabling to remedy all these shortcomings. In particular, the invention enables simplification of the tooling sets and better control of the welding process, while ensuring notably regular formation of sparks during the starting and flashing phases.
The invention therefore relates, generally speaking, to a welding installation of such type, for butt connection of metal bands running successively along a longitudinal axis, comprising a fixed frame centred on the running axis and on which is mounted a first two-jaw retaining member for clamping the downstream end, in the running direction, of a first band, a mobile frame centred on the running axis and movable parallel to the said axis with respect to the fixed frame, whereas the said mobile frame carries a second two-jaw retaining member for clamping the upstream end of a second band following the first one, means for shearing both facing parallel edges, respectively downstream and upstream, on the ends of both bands, after clamping the said bands respectively in the first and the second retaining members, whereby each edge is shorn at a preset cantilever distance with respect to the corresponding clamping jaws, means for controlling the displacement of the mobile frame toward the fixed frame in order to bring the edges of both bands closer to one another and electric power supply means with two poles connected respectively to the said bands for flash butt welding of the said bands.
According to the invention, the electric power supply means of both bands comprise a direct current generator connected to continuous modulation means of the voltage applied between the retaining members in relation to the sizes of the bands in transversal section and according to the nature of the metals to be welded.
Particularly advantageously, the direct current generator comprises a three-phase transformer whose primary circuits are supplied with three-phase alternate current and whose secondary circuits are connected, via a diode-rectifier bridge, to two supply poles into which are plugged respectively both band retaining members.
According to the preferred embodiment, the primary circuits of the three-phase transformer are connected respectively to the three phases of an alternate network via pulse-controlled thyristor devices synchronous with the alternate current in order to modulate the applied voltage.
Advantageously, the secondary circuits of the three-phase transformer are connected to both supply poles via a xe2x80x98Graetz bridgexe2x80x99 type six-phase rectifier bridge.
Thanks to these arrangements, it is possible to flash weld metal sheets whose thickness can range, for instance, from 0.8 mm to 8 mm and whose width may exceed 2,000 mm, while adjusting the voltage between the bands continuously, and without exceeding a 15V pitch voltage for larger sizes.